Re: [PATCH RFC 02/15] migration: Allow pss->page jump over clean pages

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> Commit ba1b7c812c ("migration/ram: Optimize ram_save_host_page()") managed to
> optimize host huge page use case by scanning the dirty bitmap when looking for
> the next dirty small page to migrate.
> 
> However when updating the pss->page before returning from that function, we
> used MIN() of these two values: (1) next dirty bit, or (2) end of current sent
> huge page, to fix up pss->page.
> 
> That sounds unnecessary, because I see nowhere that requires pss->page to be
> not going over current huge page boundary.
> 
> What we need here is probably MAX() instead of MIN() so that we'll start
> scanning from the next dirty bit next time. Since pss->page can't be smaller
> than hostpage_boundary (the loop guarantees it), it probably means we don't
> need to fix it up at all.
> 
> Cc: Keqian Zhu <zhukeqian1@huawei.com>
> Cc: Kunkun Jiang <jiangkunkun@huawei.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>


Hmm, I think that's potentially necessary.  note that the start of
ram_save_host_page stores the 'start_page' at entry.
That' start_page' goes to the ram_save_release_protection and so
I think it needs to be pagesize aligned for the mmap/uffd that happens.

Dave

> ---
>  migration/ram.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 381ad56d26..94b0ad4234 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2229,8 +2229,6 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>      } while ((pss->page < hostpage_boundary) &&
>               offset_in_ramblock(pss->block,
>                                  ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));
> -    /* The offset we leave with is the min boundary of host page and block */
> -    pss->page = MIN(pss->page, hostpage_boundary);
>  
>      res = ram_save_release_protection(rs, pss, start_page);
>      return (res < 0 ? res : pages);
> -- 
> 2.32.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK